This is because the mesh is triangulated when exported; alot of your faces are Ngons (faces with more than 4 vertices) and they're not flat but distorted (expecially at the bottom of the mesh)You can preview this effect with the "triangulate" modifier in blenderIt's recommended to stick to quads when modelling, as it avoids these topology problems.

It's exacly the way Elbriga says. You should stick with quads to avoid shading issues after the export.You can still try to minimize the effects of triangulation by setting specific shading groups, thus splitting and merging edges to get normal interpolation where it makes sense.In Blender you do this by setting your model shading to smooth and using an Edge Split modifier with a good treshold. Or alternatively, splitting edges manually.